Pumps can be used in industrial applications, and they can consume relatively large amounts of energy. About 15% of all electricity consumed by the industrial sector may be consumed in pumping applications. As the price of electricity rises and the desire to reduce energy consumption increases, monitoring the energy efficiency of pump systems becomes of greater interest. In order to monitor the energy efficiency or control the pump, the location of the operation point should be determined.
There are several methods to estimate the operating point of a pump with or without additional sensors. A known method is to measure directly the flow and head of the pump, which includes two or three separate sensors. In addition, there are available model-based methods, which are based on rotation speed and torque estimates of a frequency converter and pump characteristic curves.
The method that utilizes the measured head of the pump to estimate the pump operating point, later referred to as the QH curve-based method, may not be accurate at lower flow rates, where the head curve is in some cases flat or not monotonically decreasing but at high flow rates its accuracy can increase. Another model-based method for a frequency converter is a method that utilizes estimated power consumption and rotation speed for the estimation of the operation point of the pump. This method is later referred to as the QP curve-based method. This method is not applicable, when the power curve is non-monotonic, such as at high flow rates compared to the nominal flow rate of the pump. However, at lower flow rates the estimation can be more accurate. For example, the accuracy of both methods can be affected by the shape of the characteristic curves.
Both of the previously mentioned methods used in frequency converters apply the pump characteristic curves of the model of the pump. These curves are the flow rate to head curve (QH curve) and the flow rate to power curve (QP curve) of the pump. The curves are provided by the pump manufacturer, and can be available for all pumps. An example of the characteristic curves for a known centrifugal pump can be seen in FIG. 1. Specifically, FIG. 1 gives characteristic curves for a radial flow centrifugal pump with specific speed nq=30. On the left plot there are the head to flow rate (QH) and net positive suction head to flow rate curves, on the right plot there is the power to flow (QP) curve.
The estimation of operation point of the pump can be difficult in the above mentioned cases when the characteristic curves are non-monotonic.